1. Field of the Invention
The present invention relates to a sheet feeding apparatus which feeds each of stacked sheets and an image forming apparatus which has the sheet feeding apparatus.
2. Description of the Related Art
To transfer a toner image formed on a photosensitive member onto sheets in a transfer position, an image forming apparatus, such as a copying machine or a printer, has a sheet feeding apparatus which feeds each of the sheets cut in a predetermined size to the transfer position.
There has been disclosed a sheet feeding apparatus which blows air to one-end side of sheets stacked on a tray of a storage case in a conveying direction by a loosening fan so as to lift the sheets and then adsorbs the lifted sheets onto an adsorbing and conveying belt to convey them (see Japanese Patent Application Laid-Open No. 7-196187).
The amount of lifting of the sheets is different depending on their material (thickness or weight). There has been proposed a sheet feeding apparatus which controls the raising or lowering of a tray which stacks the sheets so as to position the lifting position of the uppermost sheet lifted by blowing air within a predetermined range. Whether the lifting position of the uppermost one of the lifted sheets is within the predetermined range or not is judged by a position detecting portion. When it is outside the predetermined range, the tray is raised or lowered and is then controlled such that the uppermost sheet is positioned within the predetermined range (see Japanese Patent Application Laid-Open No. 2005-272019).
There has been known control in which one of the sheet feeding start timing of the sheet feeding apparatus and the toner image forming start timing of the image forming apparatus body precedes the other. Here, the control in which the feeding start timing precedes the image forming start timing is called image forming precedence control. The control in which the image forming start timing precedes the feeding start timing is called sheet feeding precedence control.
In an image forming apparatus which performs the image forming precedence control, the image forming operation of the image forming apparatus body is enabled until there are no sheets in the sheet feeding apparatus and need not detect the remaining amount of sheets during the feeding operation. Image forming is not started at the detection of the absence of sheets. The image forming apparatus can be easily stopped.
In an image forming apparatus which performs the sheet feeding precedence control, when there are no sheets in the sheet feeding apparatus to stop the image forming apparatus, image forming has already been started in the image forming apparatus body. The recovery process of the photosensitive member (cleaning of the photosensitive member) during image forming is necessary. To avoid the recovery process, there has been proposed an image forming apparatus which arranges a sensor which detects the remaining amount of sheets in the sheet feeding apparatus and, when the remaining amount of sheets is reduced and the sensor is turned on, changes from the image forming precedence control to the sheet feeding precedence control (see U.S. Pat. No. 6,567,620).
In the sheet feeding apparatus which blows air to lift sheets, the timing at which the sensor which detects the remaining amount of sheets is turned on is changed depending on the lifted state of the sheets, thereby increasing an error of the remaining amount of sheets in the sheet feeding apparatus.